Two hours ahead, with one large caveat
Mountain to Eastern is an addition of two hours for Denver, Salt Lake City, Albuquerque and Boise, and that holds all year because those places switch to daylight saving on the same dates as the East Coast.
Arizona is the exception that ruins the rule
Most of Arizona sits geographically in the Mountain zone but does not observe daylight saving at all. Phoenix stays at UTC−7 every day of the year. Because Eastern does shift, the gap from Phoenix to New York is two hours in winter and three hours in summer. If your Mountain contact is in Arizona, the arithmetic above is wrong for roughly eight months of the year. The Navajo Nation, which extends into Arizona, does observe daylight saving, so even inside the state it is not uniform.
Scheduling from the Mountain West
A 9:00 AM start in Denver is 11:00 AM in New York, which leaves a good shared afternoon. The constraint is the East Coast end of day: 3:00 PM Mountain is already 5:00 PM Eastern. Morning slots work; late-afternoon slots do not.
Strict MST versus Mountain local time
Strictly, MST is UTC−7 and applies only from November to March; the rest of the year Denver is on MDT at UTC−6. Because Arizona uses UTC−7 permanently, “MST” genuinely means two different things depending on who is writing it. Naming the city removes the ambiguity entirely.